In my improvisational work, someone will often start with a riff and then everyone else works off of that. What that means it whoever starts (and it could be the bassist, the keyboardist or the drummer—sometimes even a melodic instrument) noodles around until they find a short phrase that they like. This, when repeated over and over, becomes the riff.
Everyone else then slowly chimes in, supporting the riff until a melodic instrument starts developing a melody over the top of everything. When you use the riff, there is basically little chordal sophistication to the music. It’s pretty much you stay in the same key all the time.
In order to get more changes in, you really have to work harder—probably starting with the melody line, and then working from there to find the chords that support each bar of the melody. That takes a little longer to figure out, so it’s not something that you usually improvise unless you’ve got a really good rhythm section (counting bass as rhythm here).
It’s easy enough to improvise melodically on top of an existing chord structure, but it’s much harder to improvise a chord structure that supports an existing melodic line. Still, it’s doable.
But riffs are the easiest of all. They’re simple—maybe two or four bars long. They repeat over and over. It’s easy to hop on them, no matter what you’re playing, and they can develop a lot of power pretty quickly. Actually, that’s their main drawback, too. It’s too easy to turn it into an out of control train that can’t be reigned back. You need really good musicians to be able to learn to listen for the energetic changes and to be able to modulate the dynamics and even to allow the melody—and sometimes the riff—to change. If you have musicians like that, you’ve got something worth holding onto.